Method and apparatus for decoding compressed and encoded digital images

ABSTRACT

A method for decoding-decompressing a compressed-encoded digital data sequence relating to at least one compressed-encoded digital image and for providing at least one respective decoded-decompressed digital image.

PRIORITY CLAIM

This application claims priority from Italian patent application No.MI2003A001079, filed May 29, 2003, which is incorporated herein byreference.

BACKGROUND ART

The present invention relates to data decoding-decompression,particularly it relates to a method for decoding-decompressing digitalimages.

Numerous algorithms are currently known for compressing and encodingboth single (or still) digital images, used for example in photographicapplications, and digital image sequences, used for example in videoapplications.

The compression-encoding algorithms make it possible to reduce thequantity of memory required for the storage of the single images orvideo sequences. These algorithms further make it possible to reduce thebandwidth resources required for the transfer of the images betweendifferent devices or for their transmission on telecommunicationsnetworks such as, for example, the Internet.

A compression-encoding algorithm transforms a digital image, or adigital video sequence, into a compressed and encoded digital datasequence, such as in the form of a bit sequence (or bit stream).

The most common and efficient compression-encoding methods currentlyused are based on an operation of transforming the images into abidimensional spatial frequency domain. Amongst these, numerous methodsuse the Discrete Cosine Transform (DCT). One example thereof are thecompression-encoding methods, for single images, compliant with theinternational JPEG (Joint Photographic Experts Group) standard and thecompression-encoding methods, for video sequences, compliant with theinternational MPEG (Motion Picture Experts Group) standard such asMPEG-1, MPEG-2, MPEG-4.

Amongst the other standards that use compression-encoding methods basedon the DCT transform, we further mention the standards H263 and H26L.

The transformation operation is followed by a subsequent processing,which reduces the informative content of a digital image or a digitalimage sequence, by operating directly in the spatial frequency domain.

After this processing aimed at reducing the informative content, theimages are encoded according to known algorithms, through methods ofentropic coding, commonly of the type including a variable length coding(VLC). Huffman coding is, for example, a particular type of entropicvariable length coding (VLC) which reduces the number of bits necessaryto represent a data set without introducing any loss of information.

The thus compressed and encoded images, or video sequences, aretransferred in the form of a bit sequence into storage devices or areremote transmitted, for example they are exchanged between multimediacommunication terminals.

The decoding-decompression process, typically inverse to that ofcompression-encoding, is aimed at the reconstruction of the digitalimages or video sequences from the compressed and encoded data sequence.For example, the decoding-decompression process is used in applicationsthat require either such digital images or such video sequences to bedisplayed on a screen.

It is known that a bit sequence, during storage or transmission, can becorrupted, that is, altered by errors.

This problem is particularly felt when the bit sequence is transmittedon radio channels, such as those used in mobile telecommunications andvideo-communications. The presence of disturbances of various kinds onthe channel may vary the value of some bits in the sequence, which mayintroduce errors.

The decoding-decompression performances of an encoded-compressed bitsequence with methods that use an entropic variable length coding (VLC)are sensitively influenced by the presence of possible errors that havecorrupted the bit sequence to be decoded.

This is due, for example, to the fact that certain errors can be suchthat a code word is erroneously interpreted as another code word with adifferent length, without the presence of an error being detected in thedecoding step. This condition may determine a loss of synchronization inthe decoding step.

In this way, even an error in a single bit may entail the loss of alarge quantity of data thus producing a significant degradation in thequality of the video or the image.

It has been observed that the state of the art decoding-decompressiontechniques, for example those compliant with the different standardscurrently existing, in the presence of errors that alter thecompressed-encoded bit sequence, do not ensure satisfactory performancesin terms of the quality of the decoded-decompressed image or sequence.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a decoding-decompressionmethod that is robust towards errors and that is able to provide betterperformance than the state of the art decoding methods.

This improvement is achieved with a method fordecoding-decompressionmethod, as described in claims 1 to 15.

A further aspect of the present invention provides a method fortransferring digital images, as described in claims 16 to 19. A furtheraspect of the present invention is a multimedia communication apparatusas described in claim 20.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and the advantages of the invention will bebetter understood from the following detailed description of the variousembodiments thereof, being non-limiting examples, in relation to theappended figures, wherein:

FIG. 1 shows a block diagram relating to a compression/encoding methodconform to the MPEG-4 standard;

FIG. 2 a shows a possible segmentation of a digital image into pixelblocks;

FIG. 2 b shows a transform of the image of FIG. 2 into a spatialfrequency domain;

FIG. 3 a shows the transform of FIG. 2 after a quantization step;

FIG. 3 b shows an enlarged detail of the block diagram of FIG. 1;

FIG. 4 shows an example of an “ESCAPE” fixed length coding;

FIG. 5 shows the structure of a video packet and the structure of amacroblock,

FIG. 6 schematically shows a block diagram relative to a firstembodiment of a decoding-decompression method in accordance with thepresent invention;

FIG. 7 shows an enlarged detail of the block diagram of FIG. 6;

FIG. 8 a shows a range of possible digital values for the DCTcoefficients and a first preset range of dequantized coefficients;

FIG. 8 b shows a range of possible digital values for the DCTcoefficients, a first preset range of dequantized DCT coefficients and asecond preset range of dequantized DCT coefficients

FIG. 9 shows a block diagram relating to an alternative embodiment of adecoding-decompression method in accordance with the present invention,and

FIG. 10 shows an experimental graph relating to the comparison ofperformance between a decoding-decompression method according to anembodiment of the present invention and a conventional method.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The following discussion is presented to enable a person skilled in theart to make and use the invention. Various modifications to theembodiments will be readily apparent to those skilled in the art, andthe generic principles herein may be applied to other embodiments andapplications without departing from the spirit and scope of the presentinvention. Thus, the present invention is not intended to be limited tothe embodiments shown, but is to be accorded the widest scope consistentwith the principles and features disclosed herein.

An embodiment of a decoding-decompression method in accordance with theinvention that will be described below will refer in a particular, butin a non-limiting way, to the decoding-decompression of a digital datasequence obtained from the compression-encoding of a video sequence inaccordance with the MPEG-4 standard.

As far as this is concerned, it should be noted that the teachings ofthe present invention are also applicable to the decoding-decompressionof data sequences compressed-encoded with techniques different from theparticular method of compression-coding (MPEG-4) to which reference ismade for embodiments of the present description.

Particularly, not only is the decoding-decompression method according toan embodiment of the invention applicable to video sequence images, butalso applicable to single (or still) images. For example, it isapplicable to single images compressed-encoded according to the JPEGstandard.

A method according to an embodiment of the present invention is, forexample, advantageously usable in new generation multimediacommunication terminals or apparatus for transmitting/receiving througha radio frequency signal, compressed-encoded single images, or videosequences. In such terminals, the images and the video sequences areacquired/stored/displayed and transmitted/received using conventionaldevices and technologies and are therefore evident to those skilled inthe art.

A video sequence comprises a series of consecutively acquired imagesthat must be viewed at a preset speed. In accordance with the MPEG-4standard, the images of a video sequence may be compressed and encodedusing three types of compression-encoding algorithms:

-   -   I-type (intra-image) compression-encoding, wherein the current        image is compressed-encoded independently from the other images        of the sequence, exploiting only the spatial correlation inside        the image;    -   P-type (predictive) compression-encoding, in which the current        image is coded with reference to an image previous thereto;    -   B-type (bidimensional) compression-encoding, in which the        current image is encoded with reference to a previous image and        a subsequent image.

A video sequence compressed-encoded according to the MPEG-4 standard maycomprise images compressed and encoded with I-type, P-type or B-typecoding interleaved with one another.

As a non-limiting example, in the present description reference is madeprincipally to the decoding-decompression of an MPEG-4 video sequence,in which the images are compressed-encoded with an I-typecompression-encoding. The teachings of the present description can beadapted to the case in which the image sequence to decode-decompressalso contains images encoded according to the MPEG-4 standard with B-and P-type coding-compression.

FIG. 1 represents a block diagram of a compression-encoding methodImg_(Enc) conforming to the MPEG-4 standard, for compressing-encoding adigital image sequence by an I-type coding.

As in this type of coding the images are compressed-encoded irrespectiveof one another, we will consider the case in which thecompression-encoding method Img_(Enc) produces, starting from a digitalimage Img_(n) pertaining to a video sequence Img₁, Img₂, . . . ,Img_(n−1), Img_(n), Img_(n+1) a digital sequence Bit_(Stream) compressedand encoded in accordance with the MPEG-4 standard.

For a more accurate and detailed description relative to the MPEG-4standard reference is in any case made to the specification “ISO/IECJTC1/SC29/WG11 N 2502: Final Draft of International MPEG-4 standard”.

As shown in FIG. 1, the compression-encoding method Img_(Enc) comprisesa first processing step TRANSF for producing, starting from the digitalimage Img_(n), coefficients of a transform in a bi-dimensional spatialfrequencies domain. In the specific case of the MPEG-4 standard (butalso in other standards such as H263, H26L, MPEG-1, MPEG-2, JPEG), thetransform used is the Discrete Cosine Transform or DCT.

For example, the digital images at input to the processing step TRANSFare in YcrCb format. As is known to one skilled in the art, thetransformation operation is independently performed on the threecomponents Y, Cr and Cb.

Furthermore, usually, the chrominance components Cr, Cb are sub-sampledbefore being subject to the transformation step.

FIG. 2 shows that in order to obtain the transform of the image Img_(n),in the processing step TRANSF the image Img_(n) is segmented into aplurality of pixel blocks B₁₁, B₁₂, . . . , B_(jk), associated toadjacent and not overlapping regions of the image Img_(n).

Commonly, such blocks are square 8×8 pixel blocks.

The transform in the bi-dimensional spatial frequency domain isperformed on these blocks, in this specific example the DCT, in order toobtain a corresponding plurality of blocks F₁₁, F₁₂, . . . , F_(jk) oftransform coefficients.

In the specific case, the coefficients of a block are DCT coefficientsand correspond to amplitudes of orthogonal waveforms that define therepresentation of the block in the bi-dimensional domain of the spatialfrequencies DCT.

Henceforth reference will be made to the transform coefficientsindicating them more simply with the term “coefficients”.

The plurality of coefficient blocks F₁₁, F₁₂, . . . , F_(jk), as awhole, constitutes a transformed image F_Img_(n).

The coefficients are digital values that, according to a preset numberof bits used for their digital representation, belong to a range [a, b]of possible digital values, in which “a” and “b” represent the extremedigital values of the range [a, b]. For example, in accordance with theMPEG-4 standard the transformed coefficients are digital values, bothpositive and negative, represented with 12 bits, therefore pertaining tothe range [−2048, 2047].

Returning to the scheme in FIG. 1, the compression-coding methodImg_(Enc) comprises a subsequent quantization step QUANT for obtainingfrom the plurality of blocks F₁₁, F₁₂, . . . , F_(jk) of transformcoefficients a corresponding plurality of quantized transformcoefficient blocks Q₁₁, Q₁₂, . . . , Q_(jk) as shown in FIG. 3 a.

The quantization step QUANT is such to perform the division, block byblock, of every coefficient block for a corresponding integercoefficient, contained in a suitable matrix, known with the name ofquantization matrix. Conventionally, the quantization matrix variesaccording to the component, which is the plane, of the image to becompressed. Besides, for blocks belonging to a same plane, thequantization matrix is obtained by a multiplication step of a samestarting quantization matrix for a suitable scalar coefficient,denominated quantization factor, which may differ from one block toanother.

Each coefficient, after the division operation, undergoes an operationof rounding up to the subsequent integer number.

In practice, the quantization operation is such to reduce the precisionused in the digital representation of the transform coefficients. Byvarying the quantization factor such precision is reduced to a greateror lesser extent.

Usually, due to the division operation and the subsequent rounding upoperation, quantization makes equal to zero many block coefficients,especially those associated to higher spatial frequencies.

Subsequently the quantized transform block coefficients Q₁₁, Q₁₂, . . ., Q_(jk) are individually subject to an entropic encoding step ENTR_Enc.

As shown in FIG. 3 b, the entropic encoding phase ENTR_Enc includes apreliminary ordering step ORD of the quantized coefficients, which isperformed block by block.

The ordering step ORD generates from each block, or matrix, Q₁₁, Q₁₂, .. . , Q_(jk) of quantized coefficients a respective vector V₁₁, V₁₂, . .. , V_(jk) of quantized coefficients, in which the coefficients aresubstantially ordered according to progressively increasing spatialfrequencies.

In this way, starting from a plurality of bidimensional blocks ormatrices of quantized coefficients, a plurality of vectors V₁₁, V₁₂, . .. , V_(jk) is produced in sequence, each one containing the quantizedcoefficients pertaining to a respective block Q₁₁, Q₁₂, . . . , Q_(jk).

Two examples of ordering used in the MPEG-4 standard are the orderingaccording to a “zig-zag” scan of the block coefficients (as in the JPEGcase) and the ordering according to a scan order known to one skilled inthe art with the name of “alternating zig-zag”. The latter has twovariants, indicated with “alternating horizontal zig-zag” and“alternating vertical zig-zag”.

The ordering step has the purpose of ordering the quantized coefficientsof a block so as to obtain a final sequence of coefficients equal tozero, in order to increase the efficiency of the subsequent coding step.

In fact, the coefficient ordering step ORD is followed by anintermediate coding step RLE_Enc with the name of RLE (Run LengthEncoding) or “run length”, in which the quantized coefficient vectorsV₁₁, V₁₂, . . . , V_(jk) are processed individually.

In this type of coding, instead of using a value for storing eachquantized coefficient of a vector V₁₁, only the values (called “levels”)of the quantized coefficients different from zero and the number (called“run”) of previous coefficients equal to zero are stored. Each paircomprising a “level” value and a “run” number constitutes an outputsymbol produced by the run-length coding.

In a final coding step VLC_FLC the symbols outputted from theintermediate processing step RLE-Enc of “run-length” coding areprocessed.

The final encoding step VLC_FLC includes a variable length codingoperation or VLC. This operation is a reversible processing step for thecoding of data and is such to assign shorter code words to statisticallymore frequently occurring symbols and longer code words to statisticallyless frequently occurring symbols.

In accordance with coding methods conforming to the JPEG standard, allthe “run-length” symbols are encoded by a variable length coding, usingstandard encoding tables.

In the final coding step VLC_FLC, as occurs in the example in the MPEG-4standard, in addition to the variable length coding operation, a fixedlength coding operation FLC may be performed.

In fact, according to certain compression-encoding methods, for examplecompliant to the MPEG-4 standard, the final coding step VLC_FLC performsa variable length coding operation for a subset of block symbols(constituted by the combination of a “run” and a “level”) having agreater statistical frequency. The variable length coding operation, forexample, associates variable length code words to symbols having agreater statistical frequency, by using standard coding tables.

The remaining symbols of the block, i.e. those that are less probable,are encoded with fixed length code words, by using a fixed length codingoperation, or FLC (Fixed Length Coding).

For example, the MPEG-4 standard provides, for less probable symbols,the use of a fixed length coding, known by the name of “ESCAPE coding”.

The structure of a code word Cod_W that belongs to the output alphabetof an ESCAPE fixed length coding operation is schematically illustratedin FIG. 4, in the case in which this word is destined to be decoded witha Reversible Variable Length Decoding (RVLD).

The first and the last field indicated with ESCAPE, in this examplecomposed by the sequence of bits 00001, are used to signal the start andend of the code word Cod_W.

The second field, indicated with LAST, is composed of one bit and isused to distinguish whether or not the code word is the last of thesequence.

The third field, indicated with RUN, in the example is a sequence of 6bits that contains the RUN value of an encoded signal with a“run-length” encoding.

Two fields, each one composed of one bit (commonly having value “1”), isused to avoid the emulation of the “marker” of re-synchronization.

Finally, the LEVEL field, composed of 11 bits, contains the values(“level”) of the quantized coefficient of the coded “run-length” symbol.In accordance with the MPEG-4 standard, the LEVEL field is constitutedby 11 bits in the case in which reversible variable length coding (knownby the abbreviation RLVD) is used, otherwise it is constituted by 12bits.

As shown in FIG. 5, in the methods in accordance with the MPEG standard,the encoded and compressed data flow Bit_(Stream) is subsequentlyorganized at a hierarchy level into Video “packets” VP. Each packet VPcomprises substantially one heading VP-head and comprises a data fieldMB_data containing data (i.e. quantized and transformed coefficients)relating to a plurality of macroblocks. The structure of a video packetis in reality more complex, but will not be further detailed because itis known to one skilled in the art.

A macroblock MB is, on the other hand, a data structure comprising thecompressed and encoded coefficients corresponding to a region of theinitial image having dimensions of 16×16 pixels. More in detail, asshown in FIG. 5, a macroblock MB comprises the quantized and encodedcoefficients obtained from the compression-encoding of 4 blocks withdimensions of 8×8 of the luminance plane Y, indicated overall by MB_Y. Amacroblock MB further comprises the quantized and encoded coefficientsobtained from the compression-encoding of two blocks with dimensions of8×8 indicated with MB_Cb and MB_Cr, pertaining respectively to thechrominance plane Cb and the chrominance plane Cr.

A slightly different, but similar condition, arises in the compressionof still images, for example in the methods conforming to the JPEGstandard.

In such methods, reference is not made to the video packets VP. However,the quantized and encoded coefficients are organized in structures,similar to the macroblocks described above, known to one skilled in theart by the name MCU (Minimum Compression Unit). The dimensions of suchstructures depend on the initial sub-sampling of the chrominance planesCr and Cb.

With reference to FIG. 6, an embodiment of a decoding-decompressionmethod Img_(Dec) in accordance with the present invention will bedescribed.

In greater detail, the particular decoding-decompression methodImg_(Dec) described below refers to the decoding-decompression of a datasequence Bit_(Stream) compressed-encoded in accordance with the MPEG-4standard, obtained for example, by an I-type compression-encodingMPEG-4, from a sequence of digital images. This means that, forsimplicity, in the present description the part of decoding that usesmotion vectors and motion compensation, used on the other hand in B-typeand P-type image decoding-decompression, will not be dealt with.

In one embodiment, the decoding-decompression method may be implementedby software resident in a telecommunications device, or it can beimplemented as a hardware device, or as a suitable combination ofhardware and software.

The decoding-decompression method Img_(Dec) comprises an initial parsingstep Rx_Parse of the compressed-encoded bit sequence Bit_(Stream). Thisstep includes a series of operations relating to the reception/readingof the compressed-encoded bit sequence. Such reception/readingoperations are known to one skilled in the art and therefore will not befurther described.

The initial parsing step Rx_Parse further includes a series of analysisoperations of the sequence received in order to identify and extractthere from initially compressed-encoded data relating to differentimages and subsequently groups of compressed-encoded data relating todifferent pixel blocks of the same image.

Particularly, in the case of the MPEG-4 standard, or more in general theMPEG standard, analysis operations are used to extract from the bitsequence received video packets VP relating for example to an imageImg_(n) and then to extract there from the macroblocks MB, also relatingto the image Img_(n). Subsequently, from the latter, the analysisoperations are used to identify and separate compressed-encoded datarelating to single 8×8 pixel blocks of the same image Img_(n).

Also the analysis operations are known to one skilled in the art and arecommonly indicated with the term “parsing”.

The decoding-decompression method Img_(Dec) comprises a subsequententropic decoding phase Entr_Dec, shown in greater detail in FIG. 7.This phase, preferably, operates on the data of the compressed andencoded bit sequence, relating to individual 8×8 pixel blocks, in orderto produce at output blocks {circumflex over (Q)}_(jk) of quantizedtransform coefficients.

Particularly, the entropic decoding step Entr_Dec includes a firstprocessing step VLD_FLD suitable to perform a variable length decodingoperation. The variable length decoding operation is performed,preferably, in a known way, through the use of a decoding table, inorder to produce at output, for each individual block, quantizedtransform coefficients preferably in the form of “run-length” symbols.

In an alternative embodiment, as occurs in decoding in accordance withthe MPEG standards, the first processing phase VLD_FLD is also suitableto perform a fixed length decoding operation, for decoding coefficientsthat in the respective blocks have a lower statistical occurrence andthus are compressed-encoded with a fixed length coding (FLC).

The fixed length decoding operation is performed preferably in a knownway, for example, through the use of an “ESCAPE” decoding, in order toprovide as output, quantized transform coefficients preferably in theform of “run-length” symbols.

In the embodiment in which from the first processing step VLD_FLDquantized transform coefficients are produced as output in the form of“run-length” symbols, the entropic decoding step Entr_Dec includes anintermediate RLD (Run Length Decoding) decoding step for producingvectors {circumflex over (V)}_(jk) comprising quantized coefficientsordered substantially according to increasing spatial frequencies.

In the embodiment represented in FIG. 7, the entropic decoding stepEntr_Dec includes a final inverse ordering step I_ORD that enables toreorder the quantized coefficients of each vector {circumflex over(V)}_(jk) in a data structure in the form of a block {circumflex over(Q)}_(jk), or matrix, of quantized coefficients. The inverse orderingoperation I_ORD corresponds to an inverse operation to that of orderingORD performed during compression-encoding.

If, during the entropic encoding step ENTR_Enc, or during thetransmission of the compressed-encoded data sequence Bit_(Stream), orthe storing thereof on a physical memory medium or, again, during theentropic decoding step Entr_Dec errors occur, the quantized coefficientblocks {circumflex over (Q)}_(jk) provided as output by the entropicdecoding step do not coincide with the corresponding blocks of quantizedcoefficients Q_(jk) provided as output from the quantization step in thecompression-encoding procedure.

Returning to the diagram in FIG. 6, the blocks of quantized coefficients{circumflex over (Q)}_(jk) outputted from the entropic decoding stepEntr_Dec are subject to a dequantization, or inverse quantization, step,indicated in the figure with I_Quant. The latter is performed accordingto conventional techniques and will not be further described.

The inverse quantization step I_Quant provides from blocks of quantizedcoefficients {circumflex over (Q)}_(jk) respective blocks of dequantizedcoefficients {circumflex over (F)}_(jk).

Due to both potential errors and the loss of information intrinsic tothe non-reversible process of quantization/inverse quantization, theblocks of dequantized coefficients {circumflex over (F)}_(jk) suppliedas output from the inverse quantization step I_Quant do not coincide,with the corresponding blocks of transform coefficients F_(jk) suppliedas output from the transformation step TRANSF in thecompression-encoding procedure Img_(Enc).

As shown in FIG. 6, the decoding-decompression method Img_(Dec) furtherincludes an error check step IQ_Check on the de-quantized coefficients.

In a embodiment, the error check step IQ_Check analizes the blocks ofde-quantized coefficients {circumflex over (F)}_(jk) supplied as outputfrom the inverse quantization step I_Quant.

In greater detail, the error check phase IQ_Check identifies and selectsthe de-quantized coefficients of each block {circumflex over (F)}_(jk)outside a preset range of digital values I_(Conc)=[L_(val), H_(val)], inwhich L_(val) represents a minimum digital value, preferably negative,and H_(val) represents a maximum digital value, preferably positive.

Preferably the pre-set range I_(Conc) is linked to the precision used,during compression-encoding, for the digital representation of thetransform coefficients.

Particularly, as shown in FIG. 8 a, in an embodiment, the preset rangeI_(Conc) includes the range I_(Enc)=[a,b] of the possible digital valuesthat can be used to represent the transform coefficients in thecompression-encoding phase.

More preferably, the preset range I_(Conc) is substantially equal to, orslightly wider than the range I_(Enc) of the possiblecompression-encoding digital values.

For example, if in the compression-encoding step, after the DCTtransformation and before quantization, for the DCT coefficients (withsign) a representation is used on 12 bits (DCT coefficients between−2048 and 2047), in the error check step IQ-Check, a control operationwill be performed in order to verify whether the dequantizedcoefficients of each block {circumflex over (F)}_(jk) pertain to therange [L_(val)=a=−2048, H_(val)=b=2047]. It has been experimentallyverified that this particular choice, that is I_(Conc)=I_(Enc), isoptimum in the case of “simple profile” decoding-decompression of MPEG-4video sequences.

If for a given block {circumflex over (F)}_(jk) a dequantizedcoefficient not within the preset range [L_(val), H_(val)] is selected,the error check step IQ_Check signals the presence of an error Ê_(jk) inthe block {circumflex over (F)}_(jk) and initiates a concealment step,indicated in FIG. 6 with Conceal, aimed at concealing the error.

If on the other hand, the presence of an error Ê_(jk) in the block ofcoefficients {circumflex over (F)}_(jk), is not signaled, the errorcheck step IQ_Check is followed by an inverse transformation stepI_Transf, of a conventional type, that transforms the block ofdequantized coefficients {circumflex over (F)}_(jk) into a correspondingpixel block {circumflex over (B)}_(jk) of the decoded and decompressedimage Îmg_(n).

The concealment step is aimed at concealing the error. This stepincludes operations aimed at replacing the information identified ascorrupted by errors with other replacement information that in some waymasks, discards or conceals the errors, thus avoiding, for example, theproduction of unpleasant artifacts in the decoded and decompressed imageÎmg_(n).

It should be pointed out that, as the detection of an error is performedwithin the spatial frequency domain, it is not possible to know theexact position of the error inside the pixel block to bedecoded-decompressed.

For this reason, the coefficient block {circumflex over (F)}_(jk) ispreferably discarded and replaced and is not subject to an inversetransformation phase.

In the case of a video sequence, the pixel block in thedecoded-decompressed image Îmg_(n) during the concealment stepcorresponding to the discarded block of coefficients {circumflex over(F)}_(jk) is preferably replaced with a motion-compensated pixel blockof the previously decoded-decompressed image Îmg_(n−1) of the videosequence.

On the other hand, in the case of the decoding-decompression of ansingle (or still) image Îmg_(n), the pixel block corresponding to thediscarded coefficient block {circumflex over (F)}_(jk) is preferablyreplaced by replacement information obtained from pixel blocks spatiallyadjacent thereto. For example, the replacement information is obtainedas an average, or as a spatial interpolation of adjacent pixel blocks.

Techniques of different types may in any case be used to substituteblocks discarded because of errors.

In one embodiment variant, in the case of a decoding-decompressionmethod for video sequences, the conceal step Conceal discards andreplaces, the entire macroblock MB containing the coefficient block{circumflex over (F)}_(jk) affected by the error.

This particular latter solution presents an advantage of discarding andreplacing an entire macroblock that due to errors present within a blockthereof, would certainly be decoded in an incorrect way.

Another advantage is due to the fact that the post-processing algorithmsof decoded-decompressed images applied subsequently to thedecoding-decompression operate, conventionally, at a macroblock level.

The same principle is applicable for the decoding-decompression ofsingle images, for example compressed according to the JPEG standard.For example, in another embodiment variant, in the case of adecoding-decompression method for single images in the conceal stepConceal aimed at concealing the error, an entire minimum compressionunit MCU containing the coefficient block {circumflex over (F)}_(jk)affected by error is discarded and replaced.

In an embodiment variant, the error check step IQ_Check detects thepresence of an error by analyzing and selecting exclusively coefficientsof the coefficient block {circumflex over (F)}_(jk) obtained from aninitial entropic fixed length decoding FLD step. In fact, the decodingof such coefficients is particularly critical in the presence of errors.

FIG. 9 shows an embodiment variant of the decoding-decompression methoddescribed above and represented in FIG. 6.

In the variant of FIG. 9 the error check step IQ_Check analizes thedecompressed coefficient blocks {circumflex over (F)}_(jk) supplied onoutput from the inverse quantization step I_Quant.

In greater detail, with reference to FIG. 8 b, in the error check stepIQ_Check a first check operation IQ_Check, is performed, identical tothat described previously, in order to select dequantized coefficientsfor each block {circumflex over (F)}_(jk) falling outside a first presetrange of digital values I_(Conc)=[L_(val),H_(val)], wherein L_(val)represents a minimum digital value, preferably negative, and H_(val)represents a maximum digital value, preferably positive.

If for a given block {circumflex over (F)}_(jk) a dequantizedcoefficient is chosen that is not included in the first preset rangeI_(Conc)=[L_(val), H_(val)], the presence of an error Ê_(jk) is signaledin the block {circumflex over (F)}_(jk) and, therefore, the concealmentstep Conceal previously described with reference to the embodiment ofFIG. 6 is performed.

If, on the other hand, the presence of an error Ê_(jk) in thecoefficient block {circumflex over (F)}_(jk), is not signaled, asubsequent clipping step Clip selects the coefficients of the block{circumflex over (F)}_(jk) to be corrected. In greater detail, theclipping step Clip selects any coefficients within the first presetrange I_(Conc) and outside a second range of digital values I_(Clip),included in said first range I_(Conc).

Preferably the clipping step Clip, performs a clipping operation,equaling each coefficient selected for the correction to the respectivenearest extreme of the second range.

The corrected dequantized coefficient block {tilde over (F)}_(jk),produced by the clipping step Clip is subsequently subject to theinverse transformation step I_Transf, which produces a correspondingcorrected pixel block {tilde over (B)}_(jk) of the decompressed-decodedimage Îmg_(n).

Experimental results have shown that the decoding-decompression methodin accordance with embodiments of the present invention providessignificant increase in performance, in terms of quality of thedecoded-decompressed images, in relation to conventionaldecoding-decompression methods.

The graph shown in FIG. 10 refers to the decoding-decompression of a 128Kb/s video sequence, containing errors and compressed-encoded inaccordance with the Simple Profile MPEG-4 standard with reversiblevariable length coding RVLC.

The abscissa shows the position of each image in the sequence. That is,FN=1 corresponds to the first image of the sequence.

In the sequence, the first image is coded with I-typecompression-encoding, the subsequent images with P-typecompression-encoding.

Two curves are traced in the graph. The first of these, indicated withC1, bears a measurement of the quality, in terms of PSNR (Peak-to-peakSignal to noise ratio), of the decoded-decompressed images obtained witha standard decoding-decompression method MPEG-4.

The curve indicated with C2, refers on the other hand to adecoding-decompression method in accordance with embodiments of thepresent invention and, more particularly, in accordance with a firstembodiment described above, that is the one indicated in FIG. 6.

The PSNR measurements made and given on the graph refer to luminancechannel Y only.

Obviously, the PSNR values that can be obtained depend on the particularstrategy used for concealing errors.

The values given in the graph, refer to the case in which a macroblockof the first image containing an error block is replaced with amacroblock having a constant gray level, whilst a macroblock of any oneof the remaining images containing a block with errors is replaced witha macroblock taken from the previous image and motion-compensated.

As shown in the graph in FIG. 10, the present invention enablesanaverage improvement to be obtained in terms of PSNR of approximately 4dB compared to a conventional decoding-decompression method.

Obviously in order to satisfy contingent and specific requirements, oneskilled in the art may make numerous modifications and variants to thedecoding-decompression method, all being within the scope of protectionof this invention, as defined by the following claims.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention.

The decoding-decompression methods according to the describedembodiments may be implemented in a variety of different types ofelectronic circuitry, and may be utilized in a variety of differenttypes of electronic systems, such as computer systems, audio/videoequipment, personal digital assistants (PDAs), and so on.

1. A method for decoding-decompressing digital data thus providing atleast one decoded-decompressed digital image corresponding to arespective initial image, the method comprising the steps of: receivinga digital data sequence obtained from a compression-encoding processingof the initial image (Img_(n)); processing the digital data sequence forobtaining a plurality of dequantized blocks, each de-quantized blockcomprising digital values corresponding to dequantized coefficients of atransform in a bidimensional spatial frequencies domain of a respectivepixel block of the initial image; selecting at least one firstdequantized block comprising at least one of said digital values outsidea first preset range of digital values, and discarding said firstselected block and providing in the decoded-decompressed image arespective pixel block comprising replacement information.
 2. The methodaccording to claim 1, wherein said step of processing the digital datasequence in order to obtain the plurality of dequantized blocks includesan entropic decoding step and a subsequent inverse quantization step,the entropic decoding step including a variable length decodingoperation.
 3. The method according to claim 2, wherein said entropicdecoding step further includes a fixed length decoding operation and inwhich part of the digital values of said dequantized blocks are obtainedfrom the variable length decoding operation and part thereof areobtained from the fixed length decoding operation.
 4. The methodaccording to claim 3, wherein said selecting step selects exclusivelydigital values obtained from the fixed length decoding operation.
 5. Themethod according to claim 1, further comprising an inversetransformation step of a second block of dequantized coefficients inorder to provide a respective pixel block in the decompressed-decodedimage, the second dequantized coefficient block comprising digitalvalues all pertaining to said preset range.
 6. The method according toclaim 1, further comprising the following steps: selecting at least onefurther digital value included in a further dequantized block, thefurther value being inside the first preset range of values and beingoutside a second preset range of digital values within said first range;processing said further block, in order to replace said further valuewith a respective closer extreme value of said second preset range. 7.The method according to claim 1, wherein said transform is the DiscreteCosine Transform.
 8. The method according to claim 1, wherein saidreplacement information of said respective pixel block is obtained frompixel blocks spatially adjacent thereto.
 9. The method according toclaim 1, wherein the digital data sequence is obtained from a JPEGcompression-encoding operation of the initial digital image.
 10. Themethod according to claim 9, wherein said first dequantized blockselected is within a Minimum Compression Unit and wherein saiddiscarding step discards said entire minimum compression unit.
 11. Themethod according to claim 1, wherein the digital data sequence isobtained from an MPEG compression-encoding operation of an initialsequence of digital images, and wherein said method provides a sequenceof decoded-decompressed digital images.
 12. The method according toclaim 11, wherein said first selected block is a block with dimensions8×8 within a macroblock 16×16 and wherein the discarding step discardsthe entire macroblock and provides in the decoded-decompressed imagefour respective pixel blocks with dimensions of 8×8 comprisingreplacement information.
 13. The method according to claim 11, whereinsaid replacement information comprises a motion-compensated pixel blockof a previous image in the sequence of decoded-decompressed images. 14.The method according to claim 1, wherein the digital data sequence isobtained by a H263 or H26L compression-encoding operation of an initialdigital image sequence.
 15. Method according to claim 1, wherein saidpreset range is substantially equal to or slightly wider than an rangeof possible digital values used for a digital representation oftransform coefficients in the compression-encoding operation.
 16. Methodfor transferring at least one initial digital image from a firstcommunication terminal, the method comprising the steps of: a)compressing said at least one digital image in said first communicationterminal by performing the following operations: segmenting the initialdigital image into a plurality of pixel blocks; performing a discretetransform of the pixel blocks into a bidimensional spatial frequencydomain, in order to obtain a corresponding plurality of coefficientblocks; performing a quantization operation of the coefficient blocks,in order to obtain a plurality of quantized coefficient blocks;performing an entropic encoding operation of the quantized coefficientblocks, encoding at least part of the coefficients by means of a fixedlength coding and obtaining a digital data sequence; b) transmittingsaid digital data sequence to the second communication terminal; c)receiving and decoding-decompressing said digital data sequence in thesecond communication terminal with a decoding-decompression methodaccording to any one of the previous claims in order to obtain at leastone respective decoded-decompressed image.
 17. The method according toclaim 16, further comprising a step of acquiring said at least oneinitial digital image by the first communication terminal.
 18. Themethod according to claim 16, further comprising a step of displayingsaid decoded-decompressed image through the second communicationterminal.
 19. The method according to claim 16, wherein saidtransmitting step transmits the digital data sequence on a radiocommunication channel.
 20. A multimedia communication apparatus fordecoding-decompressing a digital data sequence providing at least onedecoded-decompressed digital image and corresponding to a respectiveinitial image wherein the decoding-decompression of said data sequenceis performed in accordance with a method according to claim
 1. 21. Amultimedia communication apparatus for decoding-decompressing a digitaldata sequence and providing at least one decoded-decompressed digitalimage that corresponds to a respective initial digital image,comprising: a receiver circuit operable to receive a digital datasequence including a compressed and encoded initial image; adequantization circuit coupled to the receiver circuit and operable todevelop dequantized blocks, each dequantized block having digital valuescorresponding to dequantized coefficients of a transform of abidimensional spatial frequency domain of a respective pixel block ofthe initial image; a selection circuit coupled to the dequantizationcircuit and operable to select a dequantized block having the digitalvalues beyond a preset range of digital values; and a discarding circuitcoupled to the selection circuit, the selection circuit operable todiscard the selected dequantized block and to substitute a replacementblock within the decoded-decompressed digital image corresponding to theinitial image.
 22. The apparatus of claim 21, wherein the selectioncircuit is further operable to determine if an error is present in thedequantized coefficients.
 23. The apparatus of claim 21, wherein themeans discarding circuit is operable to generate the replacement blockfrom sequentially adjacent dequantized blocks.
 24. The apparatus ofclaim 21, wherein the dequantization circuit is further operable todetermine minimum and maximum digital values such that a pre-set rangeis linked to a precision defined during compression-encoding of thedigital values of dequantized.
 25. An electronic system, comprising: adecoding-decompressing circuit operable to decode-decompress a digitaldata sequence and to provide at least one decoded-decompressed digitalimage that corresponds to a respective initial digital image, thecircuit including, a receiver circuit operable to receive a digital datasequence including a compressed and encoded initial image; adequantization circuit coupled to the receiver circuit and operable todevelop dequantized blocks, each dequantized block having digital valuescorresponding to dequantized coefficients of a transform of abidimensional spatial frequency domain of a respective pixel block ofthe initial image; a selection circuit coupled to the dequantizationcircuit and operable to select a dequantized block having the digitalvalues beyond a preset range of digital values; and a discarding circuitcoupled to the selection circuit, the selection circuit operable todiscard the selected dequantized block and to substitute a replacementblock within the decoded-decompressed digital image corresponding to theinitial image.
 26. The electronic system of claim 25 wherein the systemcomprises a computer system.